1. Field of the Invention
The present invention relates to gas turbines. More specifically, the present invention relates to an apparatus and method for supporting the vane segments in the turbine section of a gas turbine.
A portion of the annular gas flow path in the turbine section of a gas turbine is formed by vane segments circumferentially arrayed around the rotor. Each vane segment is comprised of an inner and an outer shroud, which together form the boundaries of the gas flow path, and one or more vanes. In order to maintain aerodynamic efficiency, it is important that the inner and outer shrouds of adjacent vane segments be properly aligned relative to each other so that a smooth surface is provided over which the hot gas may flow. Moreover, even though the shrouds may be properly aligned at assembly, aerodynamic forces imposed on the vane segments may result in misalignment of the shrouds under operating conditions. Hence, it is important that the vane segments be adequately supported so as to resist the aerodynamic forces imposed on it.
2. Description of the Prior Art
According to one approach used in the prior art to align and support the vane segments, each vane segment is affixed at its outer shroud to a cylinder, referred to as a blade ring, which encloses the vane segments. In addition, each vane segment is aligned and supported at its inner shroud by an inner cylinder. The inner cylinder support is achieved as follows. A series of torque plates are affixed to the inner cylinder so as to enclose slotted portions of the inner shrouds. The torque plates contain a splined hole for each vane segment. A splined bushing, having an eccentric pin projecting from its face, is partially inserted into the splined hole in the torque plate so that the pin engages the slot formed in the inner shroud. However, the bushing is not inserted so far into the hole that the splines in the bushing engage the splines in the hole. A cover plate is then threaded behind the bushing to stabilize it. With the cover plate in place, a square drive on the face of the bushing opposite the pin is used to rotate the bushing so that the pin forces the vane segment into alignment. After the proper alignment is obtained, the eccentric bushing is locked in place by inserting the bushing further into the hole so that the splines are engaged. The cover plate prevents disengagement of the splines by restraining motion of the bushing in the axial direction. The cover plate is peened to the torque plate to prevent the cover plate from backing out of the hole. This scheme is disclosed in U.S. Pat. No. 4,890,978, assigned to the same assignee as the current invention.
The prior art method of aligning and supporting vane segments discussed above suffers from three drawbacks. First, alignment of the vane segments can only be done on an incremental basis since the number of positions in which the bushing can be installed is limited by the number of splines. Thus, some degree of vane segment misalignment results when, as is usually the case, the desired position of the bushing for alignment purposes does not permit engagement of the splines. Hence, it would be desirable to devise a scheme which allowed infinitely fine adjustment of the vane segment alignment.
Second, since the orientation of the pin when it enters the inner shroud slot in the correctly aligned position cannot be determined in advance, the body of the pin is round to allow engagement with the slot in any orientation. However, the round pin shape results in line contact between the pin and the slot. Line contact is undesirable because vibration of the turbine components causes minute relative motion between the pin and slot resulting in wear along the contact line, eventually the wear results in a loosening of the pin in the slot and a loss of the original alignment.
Third, once the eccentric bushing is partially installed in the hole, the assembler is not able to observe the slot in the inner shroud. Thus, rotation of the eccentric bushing and minute adjustment in the vane segment alignment to allow the pin to enter the slot must be done on a trial and error basis. As a result, assembly of the inner shroud support structure is often a time consuming and tedious procedure.
Accordingly, it would be desirable to provide an apparatus and method for aligning and supporting vane segments which (1) allows infinitely fine adjustment of the vane segment alignment; (2) provides surface contact between the load-bearing surfaces on the alignment device and the inner shroud slot; and (3) aids the assembler in his efforts to insert the pin into the slot without visual guidance.